1. Field of the Invention
At the present time, because of reliability, ruggedness and cost, transistors are utilized throughout transmitters including the power amplifiers. The transistors, however, are subject to permanent damage due to excessive currents, voltages and temperatures. The excessive currents, voltages and temperatures can occur when the output of the transmitter is not terminated with a proper load or antenna. A particular problem is presented in mobile or portable transmitters where the probability of load mismatch is high due to physical obstructions to antennas, antenna damage, or errors by service personnel. Overloading of the circuitry can occur due to the application of too much forward power to the antenna and/or too much reflected power from the antenna, in case of mismatches or the like. It is desirable, therefore, to provide a power amplifier protection circuit which operates in the event the forward and/or reflected power become too high during operation.
In single sideband radio transmitters, the problems are somewhat different than those encountered in FM type radio transmitters. Protection circuits in FM type radio transmitters can monitor the average power, voltage or current applied to the antenna or available in the power amplifiers and control the drive to the power amplifiers to maintain the output power at a predetermined average level. However, in single sideband radio transmitters the power output varies in accordance with the audio modulating signal applied to the transmitter. Thus, the power output of a single sideband radio continuously varies at an audio frequency and is extremely difficult to control since the average power may be well within the defined limits and power peaks, sufficient to damage transistors and the like, may be prevalent.
2. Description of the Prior Art
Prior art protection circuits, such as disclosed in U.S. Pat. Nos. 3,641,451 and 3,866,136, both entitled "Amplifier Protection Circuit", measure the average forward and reflected power at the antenna and are generally not capable of following audio frequency variations in the power. In general, the prior art protection circuits utilize a detector for providing a foward and reflected signal indicative of the forward and reflected power. These signals are then immediately rectified and filtered to provide a relatively constant DC signal utilized to control the power supplied by the power amplifier. In single sideband transmitters the power applied to the power amplifier is generally controlled by limiting the audio input level from the microphone or controlling the gain of an intermediate amplifier stage. These methods of limiting the output power are undesirable because nonlinearities occur which increase the intermodulation products in the transmitted signal.